1. Field of the Invention
The present invention relates to the calibration of the Abbe arm in lithographic apparatus. More particularly, the invention relates to a system for calibration of the Abbe arm in lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate;
a projection system for imaging the patterned beam onto a target portion of the substrate; and
a positioning system for moving said second object table between an exposure position, at which said projection system can image said mask portion onto said substrate, and a measurement position.
2. Description of the Related Art
The term xe2x80x9cpatterning meansxe2x80x9d should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term xe2x80x9clight valvexe2x80x9d has also been used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:
A mask held by said first object table. The concept of a mask is well known in lithography, and its includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the projection beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The first object table ensures that the mask can be held at a desired position in the incoming projection beam, and that it can be moved relative to the beam if so desired.
A programmable mirror array held by a structure, which is referred to as first object table. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.
A programmable LCD array held by a structure, which is referred to as first object table. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
The projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The illumination system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94commonly referred to as a step-and-scan apparatusxe2x80x94each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in U.S. Pat. No. 5,969,441 and U.S. application Ser. No. 09/180,011, filed Feb. 27, 1998 (WO 98/40791), incorporated herein by reference. The basic operating principle behind such a multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial metrology steps on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughout, which in turn improves the cost of ownership of the machine.
The measurement performed on the substrate at the measurement position may, for example, include a determination of the spatial relationship (in X and Y directions) between various contemplated target areas on the substrate (die, areas) and a reference marker (e.g. fiducial) located on the second object table outside the area of the substrate. Such information can subsequently be employed at the exposure position to perform a fast and accurate leveling of the target areas with respect to the projection beam; for more information see WO 99/32940 (P-0079), for example. This document also describes the preparation at the measurement position of a height map relating the Z position of the substrate surface at an array of points to a reference plane of the second object table. However the reference plane is defined by a Z-interferometer at the measurement position and a different Z-interferometer is used at the at the exposure position. It IS therefore necessary to know accurately the relationship between the two Z-interferometers.
The so-called Abbe arms AAx, AAy in a lithograph device are the distances between the surface of the substrate, when mounted on the second object table, and the axes of rotation of the second object table in Rx and Ry. (In this document, R1 denotes rotation about an axis parallel to the I-direction in an orthogonal XYZ system, where the XY plane is parallel to the substrate surface at zero tilt.) These axes are fictitious and determined by software since, in general, the tilt of the second object table about the X and Y-axes is controlled by spaced-apart Z actuators rather than by rotating it about physical pivots.
The effect of a non-zero Abbe arm in the exposure position is illustrated in FIG. 2 of the accompanying drawings. As can there be seen, if the Abbe arm AAy, for example, is non-zero, rotation of the substrate W about the Y axis by an amount dRy causes a shift in the central focal point P of the projection lens system PL on the substrate by an amount dX. Correspondingly, rotation dRx causes a shift dY. For small angles of rotation the following equations hold:
dX≈dRyxc2x7AAyxe2x80x83xe2x80x83[1]
dY≈dRxxc2x7AAxxe2x80x83xe2x80x83[2]
The Abbe arms, AAx, AAy, may also conveniently be expressed in the form (Zw-Za), where Zw is the height of the surface of the substrate in the reference system of the apparatus and Za is the height of the relevant axis of rotation in that system.
Since the rotation-invariant point of the second object table is determined by software, referring to an interferometer system which measures the position of the second object table, it may be thought that there is no difficulty in setting the Abbe arm to zero. However, the high precision requirements on the Abbe arm and the irregularities that exist between interferometer systems make it necessary to calibrate the Abbe arm with very high accuracy on initial set-up. It can be necessary to repeat the calibration after set-up because of the occurrence of drift.
A known method of determining the Abbe arms at the exposure position for calibration purposes is to expose a substrate with a series of reference marks at various tilts of the second object table. After development of the substrate, measurement of the translation of the marks in X and Y for the different tilts enables the Abbe arms to be determined. Since the Abbe arm is effectively dependent on the interferometer system, calibrations have to be done at both the measurement and the exposure positions. However, the known method cannot be used at the measurement position as no exposure device is available there. The need to develop a substrate is also time consuming.
An object of the present invention is to provide a system for calibrating the Abbe arm in a lithographic projection apparatus that avoids or alleviates the disadvantages of the prior art.
According to the present invention there is provided a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate having a surface to be exposed, such that, when held on the table, the said surface lies in a reference plane;
a projection system for imaging the patterned beam onto a target portion of the substrate; and
a positioning system for moving said second object table between an exposure position, at which said projection system can image said patterned beam onto said substrate, and a measurement position; characterized by:
a calibration system for measuring lateral displacements of a reference point in a plane of said second object table as a function of tilt, at said measurement position, wherein said calibration system comprises:
a diffraction grating mounted to said second object table;
illuminating means for generating a measurement beam of radiation and directing it to be incident on said diffraction grating so as to be diffracted thereby; and
a detector for detecting the position of said diffraction grating.
By using a calibration system for measuring lateral displacements of a reference point in a plane of said second object table as a function of tilt, it is possible to measure the Abbe arm at the measurement position. Once the Abbe arm is measured it is possible to calibrate the Abbe arm to a predetermined vertical distance from the reference plane of the second object table. Advantageously this predetermined vertical distance is set to zero, such that no lateral displacement of a reference point in said reference plane will occur with tilt of the second object table.
Preferably, said diffraction grating is an at least partially transmissive diffraction grating and said calibration system comprises a light guide for directing said measurement beam to be incident on said diffraction grating in a direction substantially independent of the tilt of said second object table.
By use of a measurement beam having an angle of incidence independent of second object table tilt, the lateral shift of a reference grating with non-zero Abbe arm can be measured independently of, or separated from, the tilt dependence of the diffracted beams from the reference grating. This is necessary because, during set-up, a detector used to measure the position of the diffraction grating is not focused and therefore the measurement of the position of the grating will show a dependency on the angle of the diffracted beams. By using a measurement beam having an angle of incidence independent of second object table tilt, this problem is circumvented.
According to a further aspect of the present invention, there is provided a method of calibrating a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate having a surface to be exposed, such that, when held on the table, the said surface lies in a reference plane;
a projection system for imaging the patterned beam onto a target portion of the substrate; and
a positioning system for moving said second object table between an exposure position, at which said projection system can image said patterned beam onto said substrate, and a measurement position, said positioning system including electronic control means having parameters defining a rotation-invariant point of the second object table; the method comprising the steps of:
measuring the position of a reference point on the surface of the second object table at different tilts;
calculating the distance between the surface of the second object table and a rotation-invariant point of the second object table;
adjusting parameters of said electronic control means included in said positioning system so that said rotation-invariant point is at a predetermined vertical distance from the reference surface of the second object table.
According to a further aspect of the present invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding patterning means capable of patterning the projection beam according to a desired pattern;
a second object table for holding a substrate having a surface to be exposed, such that, when held on the table, the said surface lies in a reference plane;
a projection system for imaging the patterned beam onto a target portion of the substrate; the method comprising the steps of:
providing a substrate provided with a radiation-sensitive later to said second object table;
providing a projection beam of radiation using the illumination system;
using said patterning means to endow the projection beam with a pattern in its cross section; and
moving the second object table to an exposure position, and projecting the patterned beam of radiation onto said target portions of the substrate; characterized by the step of:
detecting displacements of a reference point of said second object table at various angles of tilt when situated at said measurement position.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference mall be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative application any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation (e.g. at a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm), EUV, X-rays, electrons and ions.